The invention relates to radiation sensors and, more particularly, to the measurement of radioactive contaminants in ambient air, soil or other types of media.
It would often be desirable to quickly, accurately and inexpensively monitor the concentration of radionuclides found in the atmosphere, soil, or other places. Usually such contaminants consist of naturally occurring radionuclides in the atmosphere, namely, progenies of Radon-222 and Radon-220 (Thoron). In places where radioactive materials are stored, processed, or used, other radioactive contaminants may be present. The need for quick, accurate and inexpensive monitors for measuring radionuclides found in the air, soil, or attached to structures is important for such locations. In addition, homeowners and real estate purchasers are concerned about radon concentrations in the home. The National Research Council recently identified Radon in the home as an important public health problem, causing as many as 21,800 deaths annually (New York Times Feb. 20, 1998, page A 13).
The current commercial Radon measurement technique is to collect Radon in a charcoal filter that is sent to a laboratory where the gamma ray emission is counted and correlated with the equivalent number of picoCuries per liter (pCi lxe2x88x921) of alpha particles in ambient air. Such commercial procedures require placing a charcoal canister within a home or commercial building for a week, then sending the sample to a laboratory for testing and waiting for the test results. One significant disadvantage with this approach is that such measurements are not made in real time. There is a critical and long-felt need for an improved Radon progenies measurement technique.
However, Radon and Thoron cannot be directly correlated with potential damage to lung tissue. Radon itself, a noble gas, is in a neutral electronic state, not attached to aerosols, and therefore, being chemically inert it is inhaled and exhaled without consequence to the lungs. The progeny 214Po is also excluded because of its very short half-life of 10xe2x88x926 min.
EPA regulations are based on radon concentrations. Radon concentrations are used this way because this is a quantity that can be readily measured. However, radon concentration is proportional to potentially harmful alpha emitters only in cases where secular equilibrium of radon with its daughter products exists, or is at least a fixed fraction. In practice, this is never the case, because this equilibrium varies between 10% and 90% depending on location and time.
The total energy of alpha particles emitted by inhaled air is the quantity, which is most strongly connected to the potential health hazard to lung tissue. Currently, the two most frequently used measurement units for radon inhalation standards are the Working Level (WL) and the pCixe2x88x921 of radon. One WL represents the total energy emitted by alpha particles from radon daughters (218Po, 214Pb and 214Bi), and presumably absorbed by lung tissue as a consequence of one person breathing ambient air containing 100 pCi lxe2x88x921 of radon. Under these conditions, the total energy, including all the energy emitted by alpha particles trapped in the lungs after enough half-lives have passed to bring their activity to virtually zero, was calculated as 1.275xc3x97105 Mev per liter of air. See J.N. Standard Radioactivity and Health, edited by R. W. Baalman, Jr., published by the Office of Scientific and technical Information, Springfield, Va., October 1988. This definition of WL does not include Radon-222 and Po-214.
The present invention can measure either the concentration of harmful alpha emitters or the total energy delivered by them. This last parameter can be measured by making the height of the chamber depicted in FIG. 2 equal to the range in air of the biggest alpha energy in question, which is 5.5 mm, rather than 27.7 mm.
The present inventors have answered the long-felt need for an improved low cost and simple measurement technique of radioactive contaminants in the air, soil, or in buildings without suffering from the disadvantages and limitations of current commercial practices and instrumentation. The radiation monitor of the present invention comprises a series of three radiation windows on top of a pancake-shaped conductive plastic chamber, with two of the windows being removable, and a microscope and a carbon fiber electrometer protruding through the chamber""s side wall, as depicted in FIGS 1 and 2. The radiation windows further comprise a thin aluminized polyester electrically conductive polyethylene terephthalate film (mylar) window to admit alpha particles, which can be combined with a second thicker mylar window to eliminate alpha particles, as well as a thicker plastic window which eliminates alpha and beta particles but admits gamma rays, as depicted in FIGS 3-5. These three windows, or means for covering, can be advantageously combined or used alone for measuring the different forms of contamination. Thus one can observe preferentially the contributions to the measurement of alpha, beta, and gamma radiation, beta and gamma radiation, or only gamma radiation. The present invention""s configuration allows the concentration of each type of radiation to be independently determined. The covering means could also include other windows for admitting or eliminating different forms of radioactive contamination.
Using an adequate air filter paper and an air sampler, one can collect air borne contaminants and provide a sample for the measurement. All (100%) radioactive aerosol particles and positively charged radioactive ions become absorbed in the filter paper. This filter paper is then placed on top of the window of the pancake-shaped monitor, which has been charged to read zero, and the discharge time, expressed in scale units per minute of discharge time, is then observed. This discharge time is inversely proportional to the amount of radioactive material captured on the filter. In our measurements, filter paper in the shape of a disk with a diameter of 4.7 cm was used. The chamber walls of the initial prototype monitor were constructed with Vectra(trademark), which is a tissue equivalent plastic. The filter paper used was Millipore pore size 0.8 xcexcm, which is available in their catalog.
To better understand and appreciate the operation of the present invention, some details of the physics involved in measuring the alpha emissions from progeny of Radon-222 are described. The source of the Radon is Uranium, naturally occurring in the earth""s crust. The decay portion of the Uranium-Radium series, comprising Radon-222 and leading to Lead-210 is as follows: 
An analogous decay series describes the decay of Radon-220 (Thoron) into its progeny.
The decay process creates positive ions that attach to aerosol particles and these particles can be collected on filter paper by passing air through the filter for a measured period of time. FIG. 6 depicts the theoretical alpha particle decay curve of the Radon progeny, RaA+RaCxe2x80x2, which are Polonium 218 and 214, respectively, as a function of time after the collection stops and the monitor begins discharging as the alpha particles from the filter paper source enter the monitor""s chamber.
The results of the measurements are shown in FIG. 7. The curves in the figure are the voltage discharge curves of the monitor, that is, scale readings versus time for several samples collected in different locations in central New Jersey, including a private home, the inventors"" laboratory and outdoors. The monitor was calibrated in units of alpha particle disintegrations per liter of air. A calibration factor of 0.458 dpm lxe2x88x921 of air per divisions/min. was obtained. Noting that the Environmental Protection Agency (EPA) limit is 8.8 dpm lxe2x88x921 of air, the FIG. 7 alpha particle concentrations varied from a minimum level of 0.77 in an outside location to a maximum level of 24.4 dpm lxe2x88x921 in a private home""s basement. The latter value is a factor of 2.8 greater than the EPA limit. The other locations have levels slightly above and roughly equal to this limit. Thus, the present invention""s monitor covers a practical range of alpha contamination, presently of great interest and importance to the EPA and many others.
The calibration factor of 0.458 dpm lxe2x88x921 per scale division per minute (out of a total of 30 scale divisions in a prototype monitor) applies for a 15-minute sampling period of air flowing at the rate of 20 liters per minute. The sampling time is followed by a one-minute waiting period before the filter paper is placed on the monitor. This time period provides a constant and repetitive delay time for all subsequent measurements.
Accordingly, one object of the present invention is to provide a quick, accurate and inexpensive radiation monitor to measure the background of naturally occurring radioactive daughter decay products of Radon-222 and Radon-220 (Thoron).
Another object of the present invention is to provide a radiation monitor to measure the naturally occurring radioactive isotopes comprising a covering means having a plurality of radiation windows on top of a conductive plastic chamber, a microscope and a carbon fiber electrometer protruding through the chamber""s wall.
An additional object of the present invention is to make measurements of the radioactive particles penetrating through the second mylar window of the covering means, which eliminates the contribution to the measurement of the alpha particles and only beta particles and gamma rays are determined. We observed a typical discharge rate of 0.025 times the alpha rate. If the air contains only decay progeny of Radon and Thoron, then the activity decreases by a factor of 1000 within about 70 hours and thus vanishes. This time period is about seven times the half-life of the decay chain 212Pbxe2x86x92 greater than 10.6 h,xcex2eta 212Bixe2x86x92 greater than 60.6 min. alpha and beta. In contrast to this environment, when long lifetime radioactive substances such as Thorium, Uranium, Plutonium or others contaminate the air, the activity persists after 70 hours of measurement time. For example in our laboratory, 70 hours after the sample was collected, we measured a persistent 0.0029 times the activity measured immediately after the collection of the sample was terminated. Thus while the short living contamination was approximately 5 dpm lxe2x88x921, the long living contamination was 0.015 dpm lxe2x88x921.
A further object of the present invention is to provide a radiation monitor to measure the concentration of gamma rays emitted by contaminants with the covering means comprising the alpha thin mylar window with a thick plastic window placed over the alpha mylar window, which allows only gamma radiation but absorbs alpha and beta radiation. In this arrangement, the radiation monitor can be used for detecting gamma rays only.
These and other objects are accomplished by the present invention""s covering means on top of a pancake-shaped conductive plastic chamber, and a microscope and a carbon fiber electrometer protruding through the chamber""s wall. The covering means comprises a plurality of radiation windows, including a thin aluminized electrically conductive polyethylene terephthalate film (mylar) first window to admit alpha particles, which can be combined with a second thicker mylar window to eliminate alpha particles and measure beta and gamma radiation and a plastic window to form the third window. The plastic radiation window of the covering means is a much thicker plastic window to eliminate alpha and beta particles but admit only gamma rays into the chamber. Each of the three windows of the covering means can be advantageously combined or used alone for measuring the different forms of contamination.
In all embodiments, the microscope and electrometer are opposite each other within the chamber and protrude through the chamber wall, with the microscope extending outward through the chamber""s wall, being optically focused on the electrometer""s carbon fiber. In accordance with the present invention, the alpha and beta thin polyester film windows are aluminized to be electrically conductive and thin enough for the desired radiation particles from the surrounding atmosphere to enter the chamber for measurement. By advantageously including a carbon fiber, self-reading electrometer within the chamber, the user can quickly view the radon dose read-out within the chamber, in real time.